Heat reclaim system

ABSTRACT

A heat reclaim system for reclaiming waste heat from a material processing line by bleeding a portion of the heated air from one or more points on the material heat processing line and directing such heated air through an economizer containing a bank of serpentine coils. Water is pumped through the economizer coils to extract heat from the air, and the heated water leaving the economizer is piped to another heat exchanger where the reclaimed heat is extracted from the water for use in other processes. More than one economizer may be provided in each heat reclaim system for receiving bleed-off air from different points on the material heat processing line, with the economizers connected in series to obtain the highest temperature difference between the air and water. Separate waste heat reclaim systems should be provided for each material heat processing line, with separate controls for each of the heat reclaim systems to permit the heat reclaim systems to operate independently of the other systems so that any one line may be shut down without affecting the operation of the other lines.

This invention relates generally as indicated to a heat reclaim systemfor recuperating waste heat from various material heat processingoperations for use in other processes.

In the heat processing of iron ore concentrates and other materials,there is usually some form of heat recuperation to minimize fuelconsumption, but the normal methods presently being used particularly instraight grate indurating equipment still permit large amounts of heatto be wasted to the atmosphere.

Additional heat recovery is possible from both the straight grate andgrate-kiln indurating equipment used in pelletizing operations and thelike. Significant amounts of heat are lost both from the cooling andrecuperation sections of such straight grate equipment, and also fromthe cooling section of such grate-kiln equipment, and it has been foundthat by bleeding off air from these areas and directing such air tosuitable heat exchangers, the heat recovery is substantial and can beeffectively used for other purposes such as building heating,concentrate drying, preheating Aerofall Mill classification air todispel moisture from the ore grinding and classification circuit, or anyother process requiring sensible heat. Since building heating isseasonable and concentrate drying is only needed during the freezingperiod, preheating the air for the ore grinding and classificationcircuit provides maximum utilization of the reclaimed waste heat whichcan be utilized as the major source of classification air heat.

Concentrate drying also requires the use of steam, and accordingly ifthe available waste heat is used for creating the steam needed forconcentrate drying, the need for putting in a costly steam plant forthat purpose is eliminated.

In a typical pelletizing plant including several straight grateindurating lines, tremendous quantities of waste heat are exhausted tothe atmosphere. Not only is this costly from a fuel consumptionstandpoint, but the higher fuel consumption also greatly increases theparticulate load in the stack exit gases and the plant emissions ofsulfur dioxide, which is objectionable from a pollution standpoint.

It is accordingly a principal object of this invention to provide apractical and economical system for reclaiming waste heat from materialheat processing operations for effective use elsewhere.

Another object is to provide such a system for reclaiming waste heatespecially for use in heating Aerofall Mill classification air.

Still another object is to provide such a heat reclaim system witheffective controls which permit reclamation of heat from plural materialheat processing lines at the same time and shut-down of any one of thelines without affecting the rest of the system.

Still another object is to provide a heat reclaim system with effectivedew point safety control and protection against overheating.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawings setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the invention may beemployed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

FIG. 1 is a schematic diagram of a typical material heat processingsystem from which waste heat may be reclaimed utilizing the various heatreclaim systems of the present invention;

FIG. 2 is a schematic diagram illustrating a preferred form of heatreclaim system in accordance with the present invention for reclaimingwaste heat from a single material processing line;

FIG. 3 is a schematic diagram similar to FIG. 1, but showing a modifiedform of heat reclaim system in accordance with this invention for use inreclaiming waste heat from two separate processing lines;

FIG. 4 is a schematic diagram of a heat reclaim system similar to FIG. 3but showing the use of more than one heat exchanger for recovering wasteheat from the same processing line; and

FIG. 5 is a fragmentary schematic diagram of another typical materialheat processing system from which waste heat may be reclaimed utilizingthe various heat reclaim systems of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in detail to the drawings and initially to FIG. 1 thereof,there is shown by way of example a material heat processing line 1including a movable grate 2 for moving the material M to be treated fromleft to right through a furnace 4 including a series of stations orsections where the material is first dried in sections 5 and 6, thenpreheated in section 7 and fired in sections 8 and 9 and finally cooledin various stages in sections 10 and 11. A plurality of burners 12direct a flame and hot gases into the preheat and firing sections 7, 8and 9 of the furnace 4.

The cooling sections 10 and 11 are primary and secondary coolingsections, respectively, which receive and cool in that order thematerial burned in the furnace. Beneath the primary and secondarycooling sections 10 and 11 are a plurality of windboxes 13 which receivecold air from a blowing fan 14 and air distributing chamber 15 anddirect the cold air upwardly through the traveling grate 2 and the bedof material M on the grate in the primary and secondary coolingsections. As shown, the air which is forced upwardly into the primarycooling section 10 is directed to an air header 16 above the furnace 4for discharge into the furnace to provide air for combustion of the fuelintroduced by the burners 12. A hood recuperation fan 17 withdraws theair through conduit 52 from the secondary cooling section 11 which hasbeen forced upwardly through the bed of material by the blower fan 14,and a portion of this heated air is directed to the burners 12 toprovide additional air for combustion of the fuel. A portion of theheated air leaving the secondary cooling section 11 is also directed tothe downdraft drying section 6. The remainder of the heated airexhausted from the secondary cooling section 11 is ordinarily vented tothe atmosphere through the stack S as by the exhaust fan 24.

A plurality of windboxes 18 are also positioned beneath the firingsections 8 and 9 of the furnace, and a suction fan 19 in communicationwith these windboxes through chamber 20 and conduit 53 exhausts hotgases from the furnace through the bed of material. These hot gases fromthe furnace may be forced upwardly by an updraft drying fan 21 into achamber 22 for discharge through a plurality of windboxes 23 beneath theupdraft drying section 5 upstream of and immediately adjacent to thedowndraft drying section 6 for upward passage of hot gases through thegranular material on the grate prior to introduction into the preheatand firing sections of the furnace. A hood exhaust fan 24 exhausts thegases from the updraft drying section 5 and vents them to atmospherethrough the stack S.

Additional windboxes 25 are shown located beneath the downdraft dryingsection 6, preheat section 7, and a portion of the firing section 8 ofthe furnace. A waste gas fan 26 in communication with these additionalwindboxes 25 through chamber 27 and conduit 54 exhausts gases from thevarious sections 6, 7 and 8 of the furnace through the bed of materialand vents the gases to atmosphere through the conduit 36 and stack S.

Suitable dampers 28 may be provided in the air conduits 29, 29' from thesecondary cooling section 11 to the burners 12 and downdraft dryingsection 6 to control the flow of heated air in the conduits, and dustcollectors, not shown, may also be provided wherever required. A bypassconduit 37 may also be provided between the suction fan 19 and updraftdrying fan 21 to permit the fan 19 to operate independently of the fan21 and bypass the updraft drying section 5 for venting of the exhaustgases from the furnace directly to atmosphere through conduit 36 andstack S upon opening the damper 28 in conduit 37 if desired.

Although the material heat processing line 1 illustrated in FIG. 1provides for some heat recuperation to minimize fuel consumption, largeamounts of heat are still wasted to the atmosphere, particularly fromthe cooling section 11 and firing sections 8 and 9. The heat reclaimsystem of the present invention provides an effective means forrecuperating such waste heat and making good use of it, in a manner tobe subsequently fully described.

In FIG. 2 there is shown a preferred form of heat recovery system 30 inaccordance with this invention for use in recovering additional wasteheat from a material processing line such as illustrated in FIG. 1.Included in the system is the hood recuperation fan 17 of the materialprocessing line of FIG. 1 which directs a portion of the hot exhaustgases from the secondary cooling section 11 to the burners 12 to providecombustion air and also to the downdraft drying section 6 for drying thematerial prior to entering the preheat and firing sections 7, 8 and 9.Also included in the system of FIG. 2 is the updraft drying fan 21 ofFIG. 1 which forces the hot exhaust gases from the furnace through theupdraft drying section 5.

To obtain additional heat recovery utilizing the heat recovery system 30of FIG. 2, a portion of the air received by the updraft drying fan 21and hood recuperation fan 17 through conduits 53 and 52, respectively,is bled off and directed to a drop-out box 31 through conduits 57 and58, respectively where the air is mixed and combined and the pressure isequalized. The amount of air bled off may be varied as desired dependingon requirements, and in fact all of the air from one or both of the fans21 and 17 may be passed through the heat recovery system if desired. Themixed bleed-off gas temperatures from the cooling and recuperation fans21, 17 of the material processing apparatus of FIG. 1 may range from525° to 575° F with an average temperature of approximately 550° F.Since these gas temperatures and volumes are variable, an exittemperature safely above apparatus dew point (approximately 180° F) isadvisable due to the sulfur dioxide content in the waste gas.

From the drop-out box 31 the bleed air is directed to an economizer 32which the air enters at approximately 550° F. A supply fan 33 may berequired for supplying air to the economizer 32 depending upon whetherthere is sufficient positive static pressure available at the twobleed-off points A and B to overcome the added pressure drop through theheat reclaim system 30. If the lowest fan pressure of fans 17 and 21 isequal to or greater than the added pressure drop, no additional fan willbe required. However, the positive pressure of the two bleed-offsupplies through conduits 57 and 58 must be equalized by adjustableorifices 34 prior to entry into the drop-out box 31.

The economizer 32 consists of a single air pass through a deep bank ofserpentine coils 35, and water is supplied to the serpentine coils 35counterflow to the air flow. The heat absorption rate of the water isdesirably based on approximately a 300° F drop in gas temperature, whichmeans that the leaving gas temperature will be in the range ofapproximately 250° F. The exit gases from the economizer 32 are thenexhausted to the main stack S through conduit 59.

Waste heat recovery in exhaust gases containing products of fuel oilcombustion and a relatively high particulate load usually makes air toair heat exchangers best suited for this type of recovery due to therelatively low waste gas entry temperature (less than 600° F) and itsrelatively high apparatus dew point (approximately 180° F). However,because of the large amounts of waste heat to be recovered, it isphysically and economically impractical to use air as the transfermedia. Accordingly, the heat recovery system of the present inventionutilizes water as the transfer media because it is economical totransport to and from the various points where it is to be used and isalso compatible with the heat recovery system of the present invention.

A closed water system is provided for the water to the economizer 32 sothat only a small amount of make-up water is required for water lostthrough leaks, packing seals, etc. Make-up water may be supplied to thesystem from a make-up water storage tank 38 by two supply pumps 39 and40. One of the supply pumps 39 is a small capacity pump which may beoperated constantly with a bypass pressure relief valve 41 on thedischarge which returns the relieved water to the storage tank. Theother pump 40 may be much larger, having a capacity equal toapproximately 25% of the total supply pumping capacity, to provide foremergency make-up in case of a major line leak.

Small capacity pressure water filters 42 and zeolite water softeners 43are provided for filtering and treating the make-up water to correct fortotal hardness and iron content, and provision should also be made forinjecting oxygen absorbing chemicals and correcting the pH of the waterto protect or inhibit corrosion of the system.

After the water passes through the economizer 32, the water is piped tothe suction side of one or more circulating supply pumps 45 (two suchpumps being shown in parallel circuit relation to each other) which pumpthe heated water to wherever desired to permit reclamation of the storedheat from the water for such useful purposes as building heating,production of low temperature steam, and so on. However, it has beenfound that maximum utilization of the reclaimed waste heat can beobtained by pumping the heated water through an air heating coil bank 46for heating air by the water for use in other processes requiringsensible heat such as an Aerofall Mill air classification system. Thecoils in the coil bank 46 should be sized to permit the air passingthereover to extract heat equal to approximately a 250° F temperaturedrop in the supply water so that the supply water is returned to theheat reclaim economizer 32 at approximately 100° F for reheating back toapproximately 350° F.

Separate waste heat reclaim systems 30 such as shown in FIG. 2 should beprovided for each material heat processing line and made to operateindependently of the other systems to permit shut-down of any one linewithout affecting the balance of the total system. Two such waste heatreclaim systems 30 and 30' are shown in FIG. 3 each associated with adifferent material heat processing line and each including updraftdrying fans 21, 21' and hood recuperation fans 17, 17'. Of course, thecoil banks 46, 46' in the Aerofall Mill air classification circuitsshould match the capabilities of the respective economizers 32, 32', andif any of the material processing lines are shut down, the correspondingair heating coil banks and pumping thereto must be reduced to preventcoil freeze-up. The flow of heated water through the respective coilbanks 46, 46' is separately controlled by independent operation of therespective power control valves 47, 47'.

Each of the various waste heat reclaim systems 30, 30' may be energizedby the pressure and temperature of the bleed-off air through conduits57, 57' and 58, 58' from each material heat processing line. Thestarting of the fans for the material heat processing lines willenergize the water circulating pump circuits for the respective heatreclaim systems, open the automatic shut-off valves 48, 48' and startwater circulating to the economizers. Conversely, when the respectivecirculating pumps 45, 45' stop, the automatic valves 48, 48' will closeon the water circuits, and if there is a substantial drop in bleed-offair temperature below a predetermined temperature, for example 500° F,the control devices 64,64' will cause the bypass air valves 49, 49' toopen for passage of the air through bypass conduits 60, 60' and 61, 61'directly to the stack S, S' and the supply air valves 50, 50' to closethus stopping air flow through the economizers 32, 32' to provideautomatic dew point safety control.

To prevent overheating of the water in the economizers 32, 32', acombination pressure-temperature device 51, 51' may be provided in thewater line on the exit side of the economizers 32, 32' for closing theair valve 50, 50' on the air supply through conduits 57, 57' and 58, 58'to the economizers and opening the bleed-off air valve 49, 49' to thestack S, S' through conduits 60, 60' and 61, 61' to shut off the airsupply. Pressure relief valves on the economizers' water supply drumsalso prevent overpressure.

Make-up water may be supplied to each of the heat reclaim systems 30,30' from the same water storage tank 38 and supply pumps 39 and 40 shownin FIG. 2.

If the temperature of the waste gas exhaust from the downdraft dryingand preheating sections 6 and 7 of any of the material heat processinglines 1 is sufficiently above the apparatus dew point, for example, 350°F, heat may also be reclaimed from such waste gas exhaust as byproviding an additional economizer 32" in the heat reclaim system 30"for that particular material processing line. The waste gas exhaust fromthe waste gas exhaust fan 26 from which additional heat is to bereclaimed is supplied through conduit 62 to an additional economizer 32"and then returned through conduit 63 to the main stack S' as shown inFIG. 4. The water flowing through the additional economizer 32" is putin series with the associated bleed-off air economizer 32' to create thehighest temperature difference between the air and water, but the waterflow is desirably increased to maintain the same temperature differencebetween the supply and return water as in the heat reclaim systems 30associated with the other material heat processing lines 1. Theseadditional economizers 32" may be automatically controlled with safetyprotection in the same manner as previously described for the bleed-offair economizers 32, 32'.

From the foregoing, it will now be apparent that the various heatreclaim systems of the present invention are both physically andeconomically practical for use in reclaiming waste heat from the exhaustgases of various material heat processing lines by bleeding off air atdifferent points along the lines. These points will of course varydepending on the particular material heat processing line. The materialheat processing line shown in FIG. 1, for example, is of the straightgrate furnace type from which additional heat recovery is possible fromboth the cooling and firing sections and also possibly from thedowndraft drying and preheating sections as previously described.Significant amounts of heat may also be recovered from other materialheat processing lines, for example, from the air being discharged fromthe cooling section 55 of a grate-kiln 56 such as shown schematically inFIG. 5 and described in greater detail in U.S. Pat. No. 3,671,027,granted June 20, 1972, which is incorporated herein by reference. All ora portion of the cooling air which is heated by passing through the bedin the cooling section 55, rather than being vented directly toatmosphere through a stack, may be directed to a heat recovery system 30of the type previously described and illustrated for recovery of heattherefrom for other purposes. Moreover, the nature of such heat reclaimsystems is such that they have a relatively long life, and the annualfuel savings resulting from such additional heat recovery is verysubstantial. There are also some side benefits obtained from such heatreclaim systems, including a reduction in the particulate load andsulfur dioxide in the stack exit gases because of a reduction in theamount of fuel required and increased operating efficiency.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A heat reclaim systemfor reclaiming waste heat from a material heat processing linecomprising means for bleeding heated air from a plurality of points on amaterial heat processing line, an economizer containing a bank ofserpentine coils, means for circulating water through said coils, meansfor directing such heated air through said economizer in a single passto permit extraction of heat from the air by the water flowing throughsaid coils to heat the water, heat extraction means for subsequentlyextracting the heat from the water after leaving said economizer for usein other processes, and means providing a closed water system permittingcontinuous flow of the water between said economizer and heat extractionmeans, each said means for bleeding heated air from the materialprocessing line comprising a supply air conduit for passage of heatedair from a point on the material heat processing line to saideconomizer, a supply air valve for selectively opening and closing saidsupply air conduit, a bypass conduit for passage of heated air from suchpoint on such material heat processing line to a stack, a bypass airvalve for selectively opening and closing said bypass conduit, and meansfor closing said supply air valve and opening said bypass air valve forstopping air flow through said economizer in response to a drop in thebleed air temperature below a predetermined level to provide automaticdew point safety control.
 2. The system of claim 1 further comprising acombination pressure-temperature device in the water line on the exitside of said economizer for closing said supply air valves and openingsaid bypass air valves to shut off the air supply to said economizer toprevent overheating of the water in said economizer in response to thetemperature of the water reaching a predetermined high level.
 3. Thesystem of claim 1 further comprising a drop-out box for receiving thebleed air from such plurality of different points to mix the bleed airprior to passage through said economizer.
 4. The system of claim 3wherein heated air is bled off from both the updraft drying fan, hoodrecuperation fan, and recuperation fan of a straight grate furnace andmixed in said drop-out box.
 5. The system of claim 3 further comprisingadjustable orifice means in each of said supply air conduits forequalizing the pressure of such heated bleed air in said air supplyconduits prior to entry into said drop-out box.
 6. The system of claim 3further comprising a supply fan between said drop-out box and economizerfor supplying the heated bleed air from said drop-out box for passagethrough said economizer in a direction counterflow to the water passingthrough said coils.
 7. The system of claim 3 wherein said heatextraction means comprises an air heating coil bank through which theheated water is circulated after circulation through said economizer forheating air by the water for use in such other processes.
 8. The systemof claim 1 further comprising pump means for providing make-up water tosaid closed water system, said pump means including a small capacitypump which is operated constantly to supply such make-up water, and alarger capacity pump to provide for emergency make-up in case of a majorline break.
 9. The system of claim 1 further comprising fan means forsupplying such heated bleed air to said economizer, water circulatingpump means for circulating the heated water from said economizer to saidheat extraction means, automatic shutoff valve means associated withsaid water pump means, and means for actuating said water circulatingpump means and opening said shut-off valve means to cause water to flowfrom said economizer to said heat extraction means only during operationof said fan means.
 10. The system of claim 9 wherein there are aplurality of said water circulating pump means in parallel circuitrelation to each other, and automatic shut-off valve means associatedwith each of said water pump circulating means.
 11. The system of claim1 wherein heated air is bled off from the cooling section of agrate-kiln.
 12. A heat reclaim system for reclaiming waste heat from amaterial heat processing line comprising a plurality of supply airconduit means for bleeding heated air from a plurality of points on amaterial heat processing line, an economizer containing a bank ofserpentine coils, means for circulating water through said coils, adrop-out box for receiving bleed air from said plural supply air conduitmeans to mix the bleed air prior to passage through said economizer,adjustable orifice means in each of said plural supply air conduit meansfor equalizing the pressure of such heated bleed air in said supply airconduit means prior to entry into said drop-out box, means for directingsuch heated air from said drop-out box through said economizer in asingle pass to permit extraction of heat from the air by the waterflowing through said coils to heat the water, heat extraction means forsubsequently extracting the heat from the water after leaving saideconomizer for use in other processes, and means providing a closedwater system permitting continuous flow of the water between saideconomizer and heat extraction means.
 13. The system of claim 12 furthercomprising a supply fan between said drop-out box and economizer forsupplying the heated bleed air from said drop-out box for passagethrough said economizer in a direction counterflow to the water passingthrough said coils.